Low-pass Butterworth Filter

Low-Pass Butterworth Filter

A low-pass Butterworth filter having 2 poles can provide a roll-off rate of -40 dB/decade and, one having 3 poles can provide a roll-off rate of -60 dB/decade.

 Furthermore, the steeper the roll-off slope, the flatter the filter magnitude curves within the pass band.

A single-pole passive low-pass filters circuit. An active filter corresponding to the same low-pass filter. It can be shown that the

two circuits have identical transfer functions. Hence, it might seem that the op-amp is redundant.

This is not true, however. If two passive filter stages are connected together, the overall transfer function is not equal to the product of the transfer functions of the individual stages.

The reason for this apparent ambiguity is the circuit loading (interaction) that arises due to the fact that the input impedance of the second stage is not sufficiently larger than the output impedance of the first stage.

But, if two active filter stages similar are connected together, such loading errors will be negligible because the op-amp with feedback (i.e., a voltage follower) introduces a very high input impedance and very low output impedance, while maintaining the voltage gain at unity.

A two-pole, low-pass Butterworth filter We could construct a two-pole filter simply by connecting together two single-pole stages of the type shown in